This invention relates in general to low surface energy fluoropolymers and more specifically to fluorinated polyurethanes which are the condensation products of fluorinated tertiary polyfunctional alcohols and aliphatic diisocyanates.
Perfluorinated polymers have long been employed as low surface energy coatings and materials. These polymers, despite their relatively high costs, have found uses in O-rings, gaskets, diaphragms, fuel tank sealants, and coatings. The high cost of these polymers, however, has limited the use of these polymers to relatively expensive items. Much of this cost may be attributed to the expense of perfluorination. A low surface energy polymer which is not highly fluorinated would eliminate much of this expense.
The isocyanates, containing the highly unsaturated --N.dbd.C.dbd.O group, are highly reactive with a host of compounds and may also react with themselves. Reaction can occur with almost any compound possessing a hydrogen atom that may be replaced by sodium and can occur with a few other compounds having hydrogen atoms not readily replaced by sodium. In such a reaction, the hydrogen becomes attached to the nitrogen of the isocyanate and the remainder of the active hydrogen compound becomes attached to the carbonyl carbon: EQU R--N.dbd.C.dbd.O+HOR.fwdarw.RNHCOOR
In many cases this addition product is quite stable. In special cases the addition product is only moderately stable and may decompose to form the initial reactant again or may decompose to other products.
In most reactions, especially with active hydrogen compounds, the aromatic isocyanates are more reactive than are the aliphatic isocyanates. In addition, substitution of electronegative groups on the aromatic ring enhances the reactivity whereas electropositive groups reduce the reactivity of the isocyanate. As would be expected, steric hindrance on either the isocyanate or the active hydrogen compound will retard the reaction. All of the reactions are subject to catalysis by acids and by bases; certain metal compounds are exceptionally powerful catalysts. In light of the great variety of reactions possible, it is fortunate that conditions which permit highly selective control of the reactions actually occurring can usually be chosen. It is this wide range of reactions possible, plus the host of reactive materials available, combined with good control of the desired reactions that permit one to tailor make a variety of polymers.
Although most urethane coating systems are largely based on aromatic diisocyanates due to their excellent properties, they typically exhibit poor color stability when exposed to ultraviolet radiation. Additives improve their performance, but only delay the color change. It has been shown that by using aliphatic diisocyanates in place of aromatic diisocyanates such as toluene diisocyanates, polyurethane coatings with outstanding light stability could be produced. Hexamethylene diisocyanate has been used for many years in experimental programs. However, owing to its high vapor pressure, a polyisocyanate of biuret structure based on hexamethylene diisocyanate is used commercially. This compound is produced from the reaction of the diisocyanate with water. This polyisocyanate: ##STR1## still retains the aliphatic characteristics desirable for a nonyellowing coating and also has a very low vapor pressure, thus reducing the hazards of unmodified hexamethylene diisocyanate.